This invention relates to an improved method of ultrasonically consolidating layers or plies of fiber-reinforced thermosetting resin matrix composite materials and more particularly to such a method which applies the ultrasonic energy generally parallel to the surface of the layer to produce substantial shear in the plies to effect heating of the resin matrix. The invention also relates to the product made by that method.
Composite materials are becoming more and more attractive for a wide variety of uses, from aircraft and automobiles to sporting goods and toys, because of their high stiffness and strength-to-weight ratio. One type of composite material includes a combination of fibers or fibrous tows in a matrix of thermosetting resin. Typically, such a composite structure is made of a number of layers of plies of xe2x80x9cprepregxe2x80x9d tape. As used herein, a composite material means a structure composed of a plurality of plies of fiber-reinforced fabric or tape in a thermosetting resin matrix. Dry fabric with unidirectional fibers or fibrous tows or woven fibers is often precombined with thermosetting resin as a xe2x80x9cprepregxe2x80x9d. Examples include carbon, glass or graphite fibers in a staged thermosetting resin matrix. The fibers typically comprise more than 35% of the material volume. Thermoset composites generally requires that the fiber/resin plies be laid-up, debulked, and then cured. This process can take a matter of hours. Such composites are to be contrasted with thermoplastic composites which are generally faster to fabricate because there is no curing involved. The thermoplastic plies need only be heated to melt the plastic matrix and then pressed together or consolidated to the other plies before cooling. With thermosetting composites, on the other hand, heating to a high enough temperature invokes an exothermic reaction causing the molecules of the resin to cross link. Once this chemical cross linking occurs, the viscosity of the resin cannot be lowered. This is not the case with thermoplastic type resins.
As used herein, consolidation means laminating two or more plies together to form a part or structure. Good consolidation implies a low level of voids (typically less than 3%) and a shear strength of the ply interfaces after curing which approaches that of the resin matrix.
Filament winding, tape placement and tow placement are common methods for fabricating parts from fiber reinforced composites.
Filament winding involves winding a filament bundle known as a xe2x80x98towxe2x80x99, to which resin has previously been applied, around a mandrel. Multiple turns around the mandrel are used to build up the required part thickness, after which the part is cured in an oven or autoclave.
During winding, thicker parts may require intermediate consolidation or compaction steps known as xe2x80x98debulksxe2x80x99, using heat in conjunction with pressure and/or vacuum. Thick parts cured without any intermediate debulks often develop fiber wrinkling, which degrades the mechanical properties of the cured part.
In tape or tow placement, a robotic head is used to place a narrow prepreg tow or tape (typically 0.125-2 inches in width) against a tool which defines the desired part shape. Multiple layers are placed at different orientations to obtain the required ply construction and part thickness. A combination of downward pressure on the tow, applied by the head, and tack (stickiness of the tow) is required to insure the tow remains on location after placement, particularly when placing tow on concave portions of the tool.
Usually the tow, and the previously deposited ply layers, are heated to increase the tack prior to placement by the robotic head.
Current tow placement machines use separate mechanisms, placed in close proximity, to apply heat and pressure. Commonly heat is applied by a jet of hot gas directed onto the tow and pressure is applied by one or more rollers or shoes which ride against the surface of the tow. The levels of consolidation achieved in this manner are such that thick tow or tape placed parts also require intermediate debulking to prevent fiber movement or wrinkling during cure.
One obstacle to consistently achieving higher levels of consolidation with these processes is the difficulty inherent in controlling temperature. Because of the heat capacity present in a hot gas system, the temperature of the gas jet, and hence the heat input to the tow, cannot be easily modulated to allow for starts, stops or changes in advance rate of the robotic head.
Intermediate debulking typically involved applying vacuum bag along with associated bag sealants, vacuum lines, connections, etc. to the layup tool or mandrel, and transfer of the tool from the tow placement machine to an oven or autoclave where it is heated to 180-250xc2x0 F. and held under vacuum pressure for up to four hours. The part is then returned to the tape placement machine to continue the lay-up process. Current thick parts such as the V-22 spindle and the F-22 pivot shaft require numerous intermediate debulks, which adds substantial cost.
A method of applying heat and pressure which achieves high levels of consolidation during tape or tow placement, thus eliminating the need for intermediate debulks, is desired and could result in substantial cost savings. The current invention relates to such method which uses an ultrasonic horn to generate the heat and pressure required for consolidation. Further, the method has the potential, in certain cases, to replace autoclave curing with curing in an oven.
Ultrasonic devices used to heat the plies have appeal for a number of reasons. Unlike convection (hot gas), conduction (hot shoes/irons), or radiation (infrared), ultrasonic consolidation does not depend upon a thermal driver to effect energy transfer to the composite material. Ultrasonic heating is instantaneously modulatable, and it provides deep, penetrating heating in the polymeric matrix beyond mere surface heating.
Ultrasonic welding has long been used to weld or bond neat (unreinforced) plastics with no or little fiber content. Such welding is done by placing an ultrasonic horn perpendicular to two plastic layers, pressing down on the layers and energizing the horn. Obeda, U.S. Pat. No. 4,713,131, teaches joining large sheets of polypropylene plastic by overlapping the sheets of plastic and welding their edges together using an ultrasonic horn placed between the sheets. Obeda, however, teaches nothing about composite materials.
But, others have attempted to use an ultrasonic horn to fabricate composite parts. See Joining Methods for Plastic and Plastic Composites: An Overview, Vijay Stokes, Polymer Engineering and Science, Mid-October 1989, vol. 29, no. 19, p. 1310-1324, specifically pp. 1322-1324, items 168-236. These previous attempts to weld even thermoplastic composites during the lamination process, using conventional ultrasonic perpendicularly disposed horn welding techniques, have yielded disappointing results because, it is speculated, the presence of the fibers alters the energy transfer in the material. Moreover, these conventional ultrasonic welding techniques set up a compression wavefront in the material which does not transmit well through the material. In 1987, engineers at Martin Marietta attempted to use an ultrasonic horn to consolidate composite thermoplastic resin-fiber plies. The horn was placed on the top of two moving plies to be consolidated in a direction perpendicular to the plies. A range of different pressures, energy levels, and feed rates were tried. The result, however, was not satisfactory: xe2x80x9cC-Scan results have shown that attempts to produce consolidated or near-consolidated laminates have not been successful thus far . . . xe2x80x9d Sonic Assisted Process Developmentxe2x80x9d, Interim Technical Report,xe2x80x9d contract No. F 33615-86-5041, Martin Marietta Baltimore for Material Laboratory Air Force Wright labs., March 1987.
Therefore, although ultrasonic horns have been used to weld plastic sheets together and, to some extent, have been successfully used to weld thermoplastics containing up to about 35% filler (such as glass or talc), the state of the art reveals no successful methodology of fabricating thermosetting or thermoplastic resin fiber matrix composite structures wherein an ultrasonic horn is used to consolidate and debulk the individual fiber-resin plies.
It is therefore an object of this invention to provide a method of fabricating a fiber-thermosetting resin matrix composite structure.
It is a further object of this invention to provide such a method which utilizes an ultrasonic horn to consolidate the fiber-resin plies of the composite structure.
It is a further object of this invention to provide such a method which is controllable, instantly modulatable, and which does not require a large thermal differential between the device and the material.
It is a further object of this invention to provide such a method which is much less likely to cause overheating or damage to the material or detract from the consolidation quality.
It is a further object of this invention to provide such a method which heats and applies pressure simultaneously.
It is a further object of this invention to provide such a method which is faster and easier to employ and is less expensive both in execution and in the equipment required, and is extremely energy-efficient.
It is a further object of this invention to provide such a method which eliminates the repeated debulking operations required of the prior art and in which debulking occurs as the plies are laid down.
It is further an object of this invention to provide such a method which can not only debulk as the plies are laid down, but which can also advance the chemical reaction of the resin so that it approaches a condition commonly referred to as the xe2x80x98gelation pointxe2x80x99, thus making it possible to final cure parts in an oven instead of in an autoclave.
It is a further object of this invention to reduce or eliminate the repeated debulking operations associated with the prior art.
It is a further object of this invention to provide more control to the debulking process.
It is a further object of this invention to benefit from the fiber xe2x80x9cnestingxe2x80x9d or settling that occurs due to ultrasonic vibration and thus enhances the ply to ply interlaminar properties.
The invention results from the realization that plies of fiber reinforced thermosetting materials can be debulked and partially or even fully cured by applying ultrasonic energy to the upper most ply in the stack of plies to induce a shear wave in the plies, the shear wave having an energy level sufficient to reduce the viscosity of the thermosetting resin to the point where the plies can be debulked under the application of pressure but not too high an energy level to avoid full chemical cross linking of the resin so that another ply can still be chemically cross linked to the upper most ply.
This invention features a method of fabricating a thermosetting matrix, fiber reinforced composite structure. The method comprises assembling a stack of fiber reinforced thermosetting material plies, engaging an ultrasonic horn with a top surface of the upper most ply; and orienting the horn at an acute angle with respect to the top surface and energizing the horn to induce a shear wave in the plies to heat the plies. Typically, the horn is moved along the upper most ply to consolidate the plies. The energy level applied by the horn is sufficient to reduce the viscosity of the thermoset resin to the point where the plies can be debulked but not high enough to fully cross-link the resin so that another ply can still be fully cross-lined to the uppermost ply. Pressure is applied to the plies via the horn and/or a separate roller or shoe to debulk the plies.
The plies of thermosetting material typically have more than 40% fiber by volume. Final curing may take place in an oven or an autoclave.
This invention also features a thermosetting matrix, fiber-reinforced composite structure made by this novel method.
The method of this invention concerns fabricating a fiber matrix composite structure. A stack of plies of fiber reinforced, thermosetting resin-matrix material are assembled on a mandrel. An ultrasonic horn is engaged with the top surface of the uppermost ply, oriented at an acute angle with respect to the surface, and energized to induce a shear wave in the plies to consolidate and debulk the plies.